In ink-jet printing methods, droplets of ink are directly projected onto a printing medium from very fine nozzles and allowed to adhere to the printing medium to obtain printed materials on which characters or images are printed. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the printing medium, non-contact with printed characters or images, etc.
In recent years, in order to impart good weathering resistance and water resistance to the printed materials, an ink for ink-jet printing which contains a pigment as a colorant has been extensively used in the ink-jet printing methods.
In the ink using the pigment, the pigment particles are dispersed in an aqueous medium. For this reason, it is known that the ink tends to need a prolonged drying time after the printing owing to slow absorption or no absorption of liquid components thereof and therefore tends to be deteriorated in fixing properties. In addition, it is also known that the ink tends to be dried or thickened inside of nozzles in a print head owing to precipitation or aggregation of the pigment particles and therefore tends to be deteriorated in ejection properties. In consequence, it has been attempted to improve fixing properties and ejection properties of the ink.
For example, JP 2007-63493A (Patent Literature 1) discloses an ink for ink-jet printing which contains at least one resin-dispersible pigment that can be dispersed in an aqueous medium by a polymer dispersant, and a self-dispersible pigment containing at least one ionic group bonded to a surface of respective pigment particles directly or through the other atomic group, and is excellent in not only color developability, but also fixing properties, high-lighter fastness, rub fastness, quality of printed characters or images, anti-bleeding properties, etc.
JP 2006-1971A (Patent Literature 2) discloses a process for producing a water-based ink for ink-jet printing which includes the step of subjecting a water dispersion of colorant-containing water-insoluble polymer particles to heat treatment at a temperature of not lower then 40° C. In Patent Literature 2, it is also described that the ink after being produced suffers from less change in viscosity and is excellent in storage stability.
JP 2006-307130A (Patent Literature 3) discloses a pigment dispersion obtained by subjecting an aqueous pigment dispersion containing a self-dispersible pigment but containing no dispersing resin to heat treatment, which is free of change in properties such as viscosity, etc., even when allowed to stand for a long period of time, and has stable ink-jet ejection properties and is excellent in reliability.